In 1977 Princeton University psychologist Julian Jaynes (1920–1997) put forth a bold new theory about human psychology in his controversial but critically acclaimed book The Origin of Consciousness in the Breakdown of the Bicameral Mind. To support his theory, Jaynes drew on neuroscience, psychology, archaeology, ancient history, religious studies, linguistics, and the analysis of ancient texts. His theory has profound implications for our understanding of human history as well as many aspects of modern life. Jaynes was far ahead of his time, and his theory remains as relevant and influential today as when it was first published.

Jaynes asserted that consciousness, or subjective introspectable self-awareness, did not emerge far back in human evolution but is a learned ability based on metaphorical language, not genetic changes. Before consciousness developed, human behavior was governed by bicameral (“two-chambered”) mentality. In place of an internal self-dialogue, bicameral individuals experienced audiovisual hallucinations directing their actions, similar to the command hallucinations experienced by many people who hear voices today. These hallucinatory experiences were interpreted as the voices of ancestors, chiefs, divine rulers, or gods. It needs to be stressed that by “consciousness” Jaynes did not mean sensory perception, thinking, rational thought, or learning.

Numerous commentators and researchers have written about Jaynesian psychology, and it has withstood criticisms and critiques. The interested reader is advised to peruse Myths vs. Facts About Julian Jaynes's Theory as well as review a summary of the evidence.

For Jaynes, his four theoretical hypothesis were intended to be a beginning; they should be regarded as testable propositions and as a roadmap pointing in the direction for more detailed, finely-tuned research. They are not conclusions.

Hypothesis One: Consciousness Based on Language
Consciousness is a learned process based on metaphors. In this sense consciousness is shaped by historical-cultural forces, not evolutionarily designed.

Hypothesis Two: Bicameral Mentality
Before people culturally acquired consciousness, an earlier, preconscious mentality governed their behavior. This mentality, called bicameral for “two-chambered,” generated audiovisual hallucinations that were attributed to and interpreted as the commands of supernatural beings.

Hypothesis Three: Dating the Transition from Bicameral Mentality to Consciousness
Though the transition occurred at different periods in various parts of the world, in most places consciousness emerged around the end of the second millennium BCE.

Hypothesis Four: Jaynes's Neurological Model for Bicameral Mentality
Jaynes proposed a neurological model underlying an earlier, preconscious, “two-chambered” mentality. This explains the lateralization of language—specifically, the change from a preconscious bicameral mentality in which the right hemisphere generated hallucinations to a conscious unicameral mentality in which hallucinations became vestigial and obsolete.